Dialkoxyorganoboranes are versatile reagents for organic syntheses and have for example been employed in so diverse fields as in the synthesis of antibiotics, insecticides and organoborohydrides. Dialkoxymethylboranes can potentially be used for the synthesis of methyl-substituted chiral oxazaborolidines (known as MeCBS named after Corey, Bakshi and Shibata. c.f. Corey, E. J. et al., Angew. Chem. Int. Ed., 37, 1986-2012 (1998)), which are powerful enantioselective catalysts for ketone reductions. Another potential use for dialkoxyorganoboranes is in Suzuki-type coupling reactions to introduce an organo group into a molecule under formation of a new C—C-bond (Miyaura, N.; Suzuki, A., Chem Rev. 95, 2457-2483 (1995)).
U.S. Pat. No. 5,463,131 describes the preparation of dialkoxyalkylboranes by reacting excess trialkyborates with diborane in the presence of an olefin, e.g.:4B(OR)3+B2H6+6C2H4 - - - →6Et-B(OR)2 (R is alkyl)
Of course, dialkoxymethylboranes cannot be prepared by that method.
Another method for the manufacture of dialkoxyalkylboranes comprises esterification of alkylboronic acids (Brown, H. C. et al., Organometallics 2(10), 1311-1316 (1983), Brown, H. C. et al., Organometallics 2(10), 1316-1319 (1983)) or trialkylboroxins (Dahlhoff, W. V. et al., Liebigs Ann. Chem. 8, 807-810 (1990)) with an appropriate alcohol.R—B(OH)2+2R′OH - - - →R—B(OR′)2+2H2O(R—BO)3+6R′OH - - - →3R—B(OR′)2+3 H2O (R, R′ is alkyl)
Water is generated in these reactions, which very often disturbs further application of the product, even if only traces of water remain.
Water is especially detrimental to the function of the alkyl-CBS catalysts, which can be prepared from dialkoxyalkylboranes. For this reason Corey proposed the use of bis(trifluoroethoxy)alkylboranes for the synthesis of ethyl- and n-butyl-CBS derivatives to avoid the formation of water as a by-product (Corey, E. J. et al., Tetrahedron Lett. 33(29), 4141-4144 (1992)). The use of bis(dialkylamino)alkylboranes for the synthesis of alkyl-CBS catalysts has also been described (Chavant, P. Y. et al., J. Organomet. Chem. 455, 37-46 (1993), but these are quite expensive reagents.
It was one object of the present invention to provide a simple and efficient process for the preparation of dialkoxyorganoboranes. The formation of water or other by-products, that might be difficult to handle or to remove, should be avoided during the process. It was another object of the present invention to establish a process for the production of organo-oxazaborolidine catalysts (organo-CBS) using dialkoxyorganoboranes. In addition, a new and efficient method for the preparation of trialkoxyboroxins should be developed.